Two classes of polyribosomes are present in the cytoplasm of most eukaryotic cells, one bound to the membranes of the endoplasmic reticulum and the other free in the cell sap. Their protein products - in regard to final distribution - fall into three categories: a) exported from the cell, b) incorporated into cell membranes, c) retained in the cell sap. To establish how the fate of the proteins is determined, and if specific information is conferred to each class of polysomes, and to investigate finer degrees of ribosomal specialization than are now recognized, including the role of factors mediating the ribosome membrane binding, are objectives of this investigation. We shall study, electron microscopically and biochemically, aspects of organization of the endoplasmic reticulum related to the above questions - in particular, the attachment of ribosomes to microsomal membranes and the functional correlates of this attachment. We will attempt to elucidate the mechanisms involved in the transfer of protein to the interior of the cisternae of the ER and in the addition of new proteins to the ER membrane. The products of microsome disassembly (bound ribosomes and stripped membranes produced by puromycin-KCl) will be used to identify the ribosomal and membrane components involved in the ribosome-membrane junction, their relationship to the growing polypeptide, and the route through which secretory proteins cross ER membranes. We will also look for conditions to obtain a functional reassembly of rough microsomes in vitro. We will investigate the relationship between free and attached polysomes and determine if they exchange subunits during the protein synthesis cycle and how ribosomes acquire the ability to bind to membranes. We will attempt to reconstruct the three dimensional structure of the ribosome from electron micrographs, to understand its function and the details of its association with the membranes of the endoplasmic reticulum.